1. Field of the Invention
The present invention relates to a telecommunication system for transmission of information signals along power supply lines, the system comprising communication devices which each of include a microcontroller and a modem, said modem comprising an interface module which establishes a link to the power supply lines for information signals. The modem also comprises a transmission module which contains a digital modulator followed by a digital/analog converter whose output is connected to the interface module, the modem finally comprising a receiving module which contains an analog/digital converter whose input is connected to the interface module, followed by a digital demodulator.
2. Description of the Prior Art
A modem comprised in such a communication device is known from U.S. Pat. No. 4,510,611. Such known modem used for transmitting and receiving data on power supply lines comprises an interface module, a transmission module and a receiving module. The transmission module of such a device performs digital modulation of the transmit signal in the ASK mode (Amplitude Shift Keying), which consists of transmitting a fixed carrier signal at a nominal frequency when the signal to be modulated is a "1", and stopping the transmission when the signal to be modulated is a "0". The range of the authorized frequencies for this type of transmission runs from 60 to 140 Khz. The receiving module of such a device comprises an autocorrelator which checks the integrity of the received signal. This signal may have been jammed either by the presence of other information signals transmitted on the line, or by the turning on of powerful apparatus such as, for example, a washing machine, which causes a sudden variation of current in the power supply lines. If the integrity of the signal is questioned by the autocorrelator, an information signal is sent to a microcontroller which controls the modem, which microcontroller has the task of sending a signal through the modem's transmission module to the device that has initially transmitted the signal of doubtful integrity in order to request a retransmission, and this is done until the signal reaches the modem unaffected. However, this procedure lacks flexibility and may cause considerable delays in communications, particularly in the case where parasitic or, in terms of electromagnetic noise, polluting phenomena are lasting and/or frequent.